The present invention relates to an electrophotographic toner and a process for producing the same that can be applied to an electrophotographic apparatus utilizing an electrophotographic process, such as a duplicator, a printer and a facsimile machine, as well as an electrophotographic developer and a process for forming an image.
As an electrophotographic process, large number of processes have been known as described in JP-B-42-23910. In general, a fixed image is formed through plural steps in that a latent image is electrically formed on a photoreceptor utilizing a photoconductive substance by various methods, the latent image thus formed is developed with a toner to form a toner image, the toner image on the photoreceptor is transferred to a transfer material such as paper through or not through an intermediate transfer material, and the transferred image is fixed by applying heat, pressure, heat and pressure, or solvent vapor. The photoreceptor is subjected to cleaning of the toner remaining on the surface thereof by various methods depending on necessity and then is again subjected to the plural steps.
As a fixing technique for fixing a transfer image transferred to a transfer material, a heat roll fixing process is generally used, in which a transfer material having a toner image transferred thereon is put through a pair of rolls consisting of a heating roll and a pressure roll. Processes where one or both of the rolls are replaced by a belt are also known as the similar process.
In these processes, a firm fixed image can be quickly obtained with high energy efficiency, and less pollution of environments due to solvents is caused, in comparison to other fixing processes. However, because the toner image is in direct contact with the roll or the belt, offset is liable to occur, in which a part of the toner is attached to the roll or the belt at the fixing time. Particularly, in the case where the temperature of the fixing device is high, offset is liable to occur since the aggregation force of the molten toner is lowered.
On the other hand, a technique where fixing is conducted at a lower temperature is demanded to reduce the consumed energy amount, and in recent years, it is demanded to terminate electricity to the fixing device except for operation to ensure energy saving. Therefore, it is necessary that the temperature of the fixing device be instantaneously increased to the working temperature upon application of electricity. For that purpose, it is desired to reduce the heat capacity of the fixing device as possible, but in that case, there is a tendency that the fluctuation width of the temperature of the fixing device becomes larger than the conventional one. That is, the overshoot of the temperature after application of electricity is increased, and the temperature drop due to insertion of paper is also increased. Furthermore, in the case where paper having a size smaller than the width of the fixing device is continuously inserted, the temperature difference between the part where the paper is in contact therewith and the part where the paper is not in contact becomes large. Particularly, in the case where the fixing device is used in a high-speed duplicator or printer, such a phenomenon is liable to occur because the capacity of the power source is liable to be short.
Therefore, an electrophotographic toner that can be fixed at a low temperature but does not cause offset in a high temperature range, i.e., that has a broad fixing latitude, is strongly demanded.
As a method for decreasing the fixing temperature of the toner, it has been known to use a crystalline resin as a binder resin constituting the toner (as described in JP-B-4-24702, JP-B-4-24703 and JP-A-9-329917). The crystalline resin cannot be generally used because it is difficult to pulverize by a melt-kneading pulverization process, and even when it is used, the fixing temperature can be decreased, but the sufficient offset resistance cannot be always obtained. That is, the molten toner penetrates into the paper to exhibit the effect of preventing the occurrence of offset, but such a problem is caused that the molten toner excessively penetrates into the paper, so as to fail to obtain a uniform image with high density.
On the other hand, as a method for preventing offset, it has been known to use a resin having a suitable molecular weight distribution that is obtained by blending a low molecular weight polymer and a high molecular weight polymer (as described in JP-A-50-134652), and also known to use a crosslinked polymer (described in JP-B-51-23354).
However, the sufficiently broad fixing latitude described in the foregoing cannot be ensured.
When a large amount of the high molecular weight polymer or the crosslinked polymer is used as described in the foregoing, offset is difficult to occur, but the fixing temperature is increased. On the other hand, when the molecular weight of the low molecular weight polymer is decreased, or the amount thereof is increased to decrease the fixing temperature, the temperature, at which offset occurs, is lowered. While the fixing temperature can be decreased by decreasing the glass transition temperature of the binder resin used or by using a plasticizer, the blocking phenomenon occurs, in which the toner is aggregated and solidified upon storage or in the fixing device.
As a method for solving the problems, various techniques have been proposed in that a crystalline resin is not used singly as the binder resin but an amorphous resin is used in combination.
In the case where the toner is produced by the melt kneading pulverization process, it has been known that pulverization becomes easy by the presence of an amorphous component. For example, JP-A-2-79860 discloses a technique using a crystalline resin and an amorphous resin in combination, and JP-A-1-163756, JP-A-1-163757, JP-A-4-81770, JP-A-155351 and JP-B-5-44032 disclose a technique using a polymer formed by chemically bonding a crystalline resin and an amorphous resin.
However, in the case where the amount of the amorphous resin is larger than the crystalline resin, the amorphous resin forms a continuous phase, and the crystalline resin forms a dispersed phase. In this case, since the crystalline resin is covered with the amorphous resin, the problem due to the crystalline resin does not occur, but since the melting behavior of the entire toner controlled by the softening temperature of the amorphous resin, it becomes difficult to realize the low temperature fixing property. On the contrary, in the case where the amount of the crystalline resin is larger than the amorphous resin, the effect of the combination of the amorphous resin cannot be sufficiently obtained.
As has been described, in order to improve the low temperature fixing property and the offset resisting property, the melt kneading pulverization process involves difficulties in that the binder resin that is effective to the low temperature fixing property and the offset resisting property is difficult to be used, and the use of the polymer having a high molecular weight or the crosslinked structure cannot provide sufficient performance. Furthermore, it is difficult to be pulverized, and thus the particle diameter of the toner is difficult to be reduced for realizing high image quality. While a polyester resin is generally used in the melt kneading pulverization process, it is difficult to be formed in to a spherical form since it is once melted and then subjected to polycondensation.
In order to reduce the amount of the non-transferred toner remaining on a photoreceptor after transfer for electric power saving, it is preferred that the toner particles are formed into a spherical form.
As a process for producing a toner for solving the problems, a wet production process, such as a particle production process by polymerization including the suspension polymerization process described in JP-B-36-10231, has been proposed.
According to the suspension polymerization process as the wet production process, the shape of the toner particles can be controlled to easily produce toner particles that are difficult to be kneaded and pulverized, and the particle size distribution can be controlled in the step of production of the particles. Therefore, the classification step is not necessarily provided, which has been necessary in the melt kneading pulverization process to make the particles uniform.
However, in the suspension polymerization process using a crystalline resin, a colorant is difficult to be dispersed in the toner, and thus a toner having a colorant suitably dispersed therein often cannot be obtained. When the colorant is aggregated in the toner, light scattering becomes conspicuous to cause problems in that the transparency and the coloration are poor.
A process for producing toner particles by the wet production process is disclosed in JP-A-10-39545 and JP-A-10-48890, in which an emulsified latex containing a sodium sulfonated polyester and a pigment dispersion are mixed with applying a shearing force, to which a halogenated alkyl is added thereto, followed by heating, and then the mixture is aggregated to cause unification, so as to produce toner particles. According to the process, while the dispersion state of the pigment can be maintained good, the fixing temperature has to be high from the standpoint of practical use.
As described in the foregoing, in order to simultaneously decrease the fixing temperature and prevent the occurrence of offset, contradict characteristics are demanded as the properties of the electrophotographic toner.
Therefore, an electrophotographic toner having a broad fixing latitude that can be fixed at a low temperature and does not cause offset in a higher temperature range have not yet been provided at the present time. Furthermore, an electrophotographic toner having a broad fixing latitude that provide the low temperature fixing property and an excellent in offset property, and exhibits good pigment dispersion has not yet been provided.
The invention has been made to solve the problems associated with the conventional techniques and to provide an electrophotographic toner that is excellent in dispersion property of a colorant and excellent in fixing property at a low temperature.
The invention has also been made to provide an electrophotographic toner having a broad fixing latitude that is good in offset resisting property.
The invention has also been made to provide a process for producing an electrophotographic toner, by which the electrophotographic toner having the excellent properties, particularly an electrophotographic toner having a spherical form.
The invention has also been made to provide an electrophotographic developer and a process for forming an image using the electrophotographic toner having the excellent properties.
As a result of earnest investigations made by the inventors to solve the problems, the following findings have been obtained, and the invention has been accomplished.
(1) When a crystalline polyester containing a carboxylic acid of two or more valences having a sulfonic acid group as a copolymerization component is used as a main component of binder resin, the dispersion property of a colorant upon production process of a toner is excellent, and a uniform toner can be produced.
(2) In order to improve the offset resisting property in a broad range of temperature with maintaining the low temperature fixing property, a crystalline resin is useful, and in order to avoid the known problem associated with the crystalline resin, i.e., excessive penetration into paper, without affecting other characteristics, it is useful to use a crosslink type crystalline resin having an unsaturated double bond as the binder resin, by which a crosslinked structure can be introduced in the production process of the toner.
(3) According to a process, in which a sodium sulfonated crystalline polyester is formed into an emulsified latex, and the latex is aggregated and unionized (unified) to produce toner particles, both the low temperature fixing property and the offset resisting property can be improved, and the dispersion property of the colorant is good, so as to produce toner particles.
According to an aspect of the invention, the electrophotographic toner contains at least a binder resin and a colorant, the binder resin containing a crystalline polyester containing a carboxylic acid of two or more valences having a sulfonic acid group as a copolymerization component.
In the invention, it is preferred that the crystalline polyester containing a carboxylic acid of two or more valences having a sulfonic acid group as a copolymerization component is crosslinked by a chemical bond, and it is more preferred that it is crosslinked by a radical chemical bond through an unsaturated bond group.
The electrophotographic toner of the invention preferably has, at an angular frequency of 1 rad/sec and a temperature of 30xc2x0 C., a storage modulus GL(30) of 1xc3x97106 Pa or more and a loss modulus GN(30) of 1xc3x97106 Pa or more, and preferably has a melting point in a temperature range of from 45 to 110xc2x0 C.
When common logarithm of a storage modulus is plotted against a temperature, the electrophotographic toner of the invention preferably satisfies the following formula (1):
|logGL(Tm+20)xe2x88x92logGL(Tm+50)|xe2x89xa61.5 xe2x80x83xe2x80x83(1) 
wherein GL(Tm+20) is a storage modulus at a temperature (Tm+20xc2x0 C.) higher than a melting point Tm by 20xc2x0 C., and GL(Tm+50) is a storage modulus at a temperature (Tm+50xc2x0 C.) higher than a melting point Tm by 50xc2x0 C.
According to another aspect of the invention, the process for producing an electrophotographic toner produces the electrophotographic toner of the invention, and the process contains a step of emulsifying a crystalline polyester containing a carboxylic acid of two or more valences containing a sulfonic acid group as a copolymerization component, and a step of aggregating and unifying the same to adjust a diameter of the toner.
According to a further aspect of the invention, the electrophotographic developer contains a carrier and a toner, the toner being the electrophotographic toner of the invention.
According to a still further aspect of the invention, the process for forming an image contains a latent image forming step of forming an electrostatic latent image on a surface of a latent image holding member, a developing step of developing the electrostatic latent image formed on the surface of the latent image holding member with a developer retained on a developer holding member to form a toner image, a transferring step of transferring the toner image formed on the surface of the latent image holding member to a surface of a transfer material, and a fixing step of heat fixing the toner image transferred to the transfer material, the developer being the electrophotographic toner of the invention or the developer of the invention.